From the Openssh 3.8 changelog (http://www.openssh.com/txt/release-3.8):
* ssh(1) now uses untrusted cookies for X11-Forwarding.
Some X11 applications might need full access to the X11 server,
see ForwardX11Trusted in ssh(1) and xauth(1) for more information.
Maybe thats the reason for the problems.
Bernd